There is known so-called steer by wire system for a steering device of a vehicle. The steer by wire system drives a turning mechanism by a motor without mechanical coupling between a steering wheel and the turning mechanism. That is, turning is performed in accordance with an operation of steering by electrically controlling the motor based on detection results of an operation direction and a steering angle of steering and by adding a turning angle which corresponds to the steering angle of steering to the turning mechanism.
Such a steering device having the steer by wire system includes a clutch mechanism that connects and disconnects the steering wheel and the turning mechanism. That is, when the steering device functions as the steer by wire system, the clutch mechanism is released, and when the steering device functions as a normal electrically operated power steering device, the clutch mechanism is connected.
Japanese Unexampled Patent Application Publication No. 2007-203885 discloses a steering device having a steer by wire function, the device including a unit that determines whether a clutch mechanism is in a connection state or a release state when the clutch mechanism is erroneously connected.
Meanwhile, when the steering device functions as the steer by wire system, feedback control is executed for the turning angle based on the detection value of the steering angle etc. In the steer by wire system, the ratio of the turning angle to the steering angle (angle ratio) is increased when a vehicle travels at lower vehicle speed, so that the turning angle is largely changed although the steering wheel is operated only slightly (quick ratio). In contrast, the angle ratio is decreased when the vehicle travels at higher vehicle speed, so that the turning angle is slightly changed although the steering wheel is operated by a large angle (slow ratio).
In this case, if the clutch mechanism is erroneously connected due to biting of a foreign substance (see Japanese Unexampled Patent Application Publication No. 2007-203885), steering is performed with an angle ratio (mechanical angle ratio) mechanically determined by the mechanism of the steering device, instead of an angle ratio set by a control system of steer by wire.
If the steer by wire system is controlled during erroneous connection of the clutch mechanism, the motor is operated to compensate the deviation between the quick ratio and the mechanical angle ratio. However, the deviation is not compensated but increased as long as the clutch mechanism is erroneously connected. This causes that, since the turning mechanism and the steering wheel are connected, the steering wheel is unintentionally rotated (self steer), and thus the driver may feel uncomfortable.
In contrast, if the clutch mechanism is erroneously connected with the slow ratio rather than the mechanical angle ratio, an actual turning angle exceeds a target turning angle of wheels set by the steering wheel. The motor is operated in order to have the actual turning angle meet the target turning angle. This causes that the steering wheel is locked (steering lock), and hence the driver may feed uncomfortable.